Modelling fluidized bed combustion of high-volatile solid fuels

Abstract

A model of an atmospheric bubbling fluidized bed combustor operated with high-volatile solid fuel feedings is presented. It aims at the assessment of axial burning profiles along the reactor and of the associated temperature profiles, relevant to combustor performance and operability. The combustor is divided into three sections: the dense bed, the splashing region and the freeboard. Three combustible phases are considered: volatile matter, relatively large non-elutriable char particles and fine char particles of elutriable size. The model takes into account phenomena that assume particular importance with high-volatile solid fuels, namely fuel particle fragmentation and attrition in the bed and volatile matter segregation and postcombustion above the bed. An energy balance on the splashing zone is set up, taking into account volatile matter and elutriated fines postcombustion and radiative and convective heat fluxes to the bed and the freeboard. Results from calculations with a high-volatile biomass fuel indicate that combustion occurs to comparable extents in the bed and in the splashing region of the combustor. Due to volatile matter segregation with respect to the bed, a significant fraction of the heat is released into the splashing region of the combustor and this results in an increase of the temperature in this region. Extensive bed solids recirculation associated to solids ejection/falling back due to bubbles bursting at bed surface promotes thermal feedback from this region to the bed of as much as 80–90% of the heat released by afterburning of volatile matter and elutriated fines. Depending on the operating conditions a significant fraction of the volatile matter may burn in the freeboard or in the cyclone.